Evolution across niches – the symmetrical case

Okie et al. (2013) provides estimates of the maximum body mass in several mammalian clades over the past 60 million years of evolution. For five of these I was able to estimate the body mass exponent for the rββ/rα-ratio and the rate of change in mass (dw/dt, Table 1, from Witting, 2016). In agreement with unconstrained natural selection across ecological niches, the exponents of the estimated rββ/rα-ratios were around one in four of the five cases. This was found to hold for both 2D evolution in even-toed ungulates and carnivores (with dw/dt exponents around 9/8) and 3D evolution in whales and primates (dw/dt exponents around 5/4). The last clade (trunked mammals; 2D) had a rββ/rα-ratio exponent around zero; which is expected for a fast body mass evolution where the increase in resource handling/density is outrunning the increase in the pre-mass component of mass specific metabolism.

Table 1 dw/dt-exponents, rββ/rα-ratios, and spatial dimensionality (d) for the evolution of the maximum mass in five mammalian clades. From Witting (2016), with data from Okie et al (2013).

In Fig. 2 I show the estimated trajectory for the maximum mass and lifespan for 3D evolution in whales. Given a 70 year lifespan of a 100 tonne blue whale today, it is estimated that the 410 kg whale ancestor that lived 31 million years ago had a lifespan around 275 years.

Fig. 2 The lifespan (τ, left) and maximum body mass (w, middle) across whales, as estimated for data provided by Okie et al (2013). Fossil whales have a dw/dt-exponent around 1.28 (right), and this coincides with a rββ/rα-ratio around one as predicted for across niche evolution with unconstrained symmetrical selection on resource handling and metabolic pace. From Witting (2016).